Prone
Shooting flat on the ground, rifle supported by bipod or rest. The most stable unsupported precision position.
Prone reduces every degree of freedom that matters: heart rate transmits to the rifle less, NPOA is bone-supported, breath sway is minimal. Most precision rifle competition prone stages assume the shooter has spent thousands of dry-fire reps in this position.
Build it from the ground up: bipod loaded forward, rear bag squeezed for fine elevation, body straight behind the rifle, cheek weight on the stock. Done well, you should be able to dry fire repeatedly without the reticle moving off the target.